Example: I have a view controller and get rid of it. But there's still an variable holding it's memory address. Accessing that results in EXEC_BAD_ACCESS. Of course. But: Is there any way to check if that variable is still valid? i.e. if it's still pointing to something that exists in memory?
You need to read this again:
Cocoa Memory Management Guidelines
In short, if you want something to stick around you must retain it.
If you want something to go away and you have previously retained it, you must release or autorelease it.
You must never call dealloc directly (except [super dealloc]; at the end of every one of your dealloc methods).
You must never release or autorelease an object that you did not retain.
Note that some methods do return retained objects that you must release. If you alloc an instance of a class, that implies a retain. If you copy and instance, the copy is retained.
If you are ever tempted to use the retainCount method, don't. It isn't useful. Only consider retain counts as a delta; if you add, you must subtract, but the absolute value is an implementation detail that should be ignored.
(In other words, even if there were ways to check for an object's validity definitively -- there aren't -- it would be the wrong answer.)
Oh, and use the Build and Analyze feature in Xcode. It does a very good -- but not quite perfect -- job of identifying memory management problems, amongst other things.
That's what the entire memory management model is set up for - if you call retain at the right times, and release and autorelease at the right times, that can't happen. You can use NSZombie to help you debug.
Use "NSZombieEnabled" break point.
For this reason only all strongly recommend us to use accessors. If your object is released anywhere, it will get assigned to nil, and there will be no harm if you call any API or method on Nil object. So please make a habit of using Accessors.
you just add this NSZombieEnabled Flag as an argument to your application in build settings. and enable it. Now you run your application in debug mode. If any such crash is about to occur, this breakpoint will show you which object is freed and where it is crashing.
Cheers,
Manjunath
If by variable, you mean whether the pointer to your object still references valid memory then:
MyClass *myVariable = [[MyClass alloc] init];
//Tons of stuff happens...
if (myVariable != nil)
//Do more stuff
Related
Reading source code of my current project, I see:
[self retain]
in one class, in its init method.
I don't understand exactly the reason.
Reading memory management rules from Apple, I don't see anything about this, and I don't see any hypothetical [self release].
The object is asserting ownership of itself when it is initialised. The only reason I can see that this might be useful is if the object needs to guarantee its own existence until some event has happened. For example, it might run a timer and then release itself when the timer expires.
If it's not documented and there is no corresponding release, it's probably a bug.
Best guess is that the person writing the code had a retain error and this was a "quick fix" around the real problem.
This seems to be probably an error, usually it's not up to the object to retain himself.
I see only one special case: delegate and notification, where you have to be much more careful about your life cycle, but even if this case, release/retain should not be done in the object itself.
Note to Erick:
In case of UIAlert, you can release it before it has been destroyed because the view has been but in the view hiercarchy, and then referenced. So the view will be automatically destroyed when it will be removed from the view hierarchy
It's not wrong to retain self. But seeing it in an init method sounds suspicious. I can't think of a good example of where that would be a good thing to do. Also though, with ARC, you can't even do that so you'd have to think of a better way of doing things. In general, if an object is required to be alive then there would be another object that is holding onto it. Self retaining objects are prone to errors where they will never be released.
If I recall correctly some classes use the self-retain to avoid pre-mature releasing. I would say it's not exactly best practice, but if you know the rules of the game (in this case Obj-C) you can break them (sometimes).
if you have some object, it's like it have healts/ lives. when you created it , it have one live. and. function 'retain' increasing his number of lives +1, release function decreasing his number of lives -1, dealloc decreasing too, alloc increasing
I am testing the following code below. ffv is declared in the interface file.
ffv = [[FullFunctionView alloc] initWithFrame:self.view.bounds];
NSLog(#"%i", [ffv retainCount]); // prints 1
[self.view insertSubview:ffv belowSubview:switchViewsBtn];
NSLog(#"%i", [ffv retainCount]); // prints 2
[ffv release]; // you can release it now since the view has ownership of ffv
NSLog(#"%i", [ffv retainCount]); // prints 1
if (ffv == nil)
NSLog(#"ffv is nil");
// "ffv is nil" is not printed
[ffv testMethod]; // "test method called" is printed
this is my [ffv testMethod] implementation
- (void)testMethod
{
NSLog(#"test method called");
}
What I deduce in this case is that even if you release an object with retain count 2, you lose ownership of that object however, the reference is still kept.
Now, my question are:
Is my deduction correct?
Is there anything else important that can be deduced from this?
What are the complications caused by still keeping (using) ffv and calling methods from ffv? (My opinion is that this is ok since the view will always own ffv and won't release it until someone calls viewDidUnload. And as long as I don't pass ffv's reference to other objects.)
There are a couple of problems with using ffv after you have released it and it's only retained by your view controller's view.
1) It introduces a potential for future bugs, because later you might not remember that ffv is otherwise not retained. When you release the view (e.g. by replacing it with another view), you have a dangling pointer that you still hold a reference to.
2) In the special case of a UIViewController the view could be released at any time (you usually never call viewDidUnload yourself). The default behavior of UIViewController, when receiving a memory warning and the view is currently not visible, is to release the view, so unless you set the reference to nil in viewDidUnload, you have a dangling pointer again, even though you never explicitly released the view yourself.
1) Is my deduction correct?
Your deduction is correct. The Memory Management Programming Guide explains that each object has one or many owners. You own any object you create using any method starting with alloc, new, copy, or mutableCopy. You can also take ownership of an object using retain. When you're done with an object, you must relinquish ownership using release or autorelease.
Releasing the object doesn't change the value of any variables that reference that object. Your variable contains the object's memory address until you reassign it, no matter what retain count the object has. Even if the object's retain count goes to zero, causing the object to get deallocated, your variable will still point at that same address. If you try to access the object after it's been deallocated, your app will normally crash with EXC_BAD_ACCESS. This is a common memory management bug.
2) Is there anything else important that can be deduced from this?
Nothing comes to mind.
3) What are the complications caused by still keeping (using) ffv and calling methods from ffv? (My opinion is that this is ok since the view will always own ffv and won't release it until someone calls viewDidUnload. And as long as I don't pass ffv's reference to other objects.)
When you call release, you are telling the Objective C runtime that you no longer require access to the object. While there may be many cases like this one in which you know the object will still exist, in practice you really shouldn't access an object after calling release. You'd just be tempting fate and setting yourself up for future bugs.
I personally don't like peppering my code with release statements, because I don't trust myself to remember them 100% of the time. Instead, I prefer to autorelease my variables as soon as I allocate them like this:
ffv = [[[FullFunctionView alloc] initWithFrame:self.view.bounds] autorelease];
This guarantees that ffv will exist at least until the end of the method. It will get released shortly thereafter, typically before the next iteration of the run loop. (In theory this could consume excessive memory if you're allocating a large number of temporary objects in a tight loop, but in practice I've never encountered this case. If I ever do, it will be easy to optimize.)
The object is not deallocated until the retain count goes to 0. As long as it's not deallocated, you can keep using it without trouble. By retaining it you ensure that it won't be deallocated under your feet; however, if you retain another object that you know retains the first object, you can get away with this form of "indirect retaining". Don't complain when you move things around later and things start breaking, though.
if (ffv == nil)
NSLog(#"ffv is nil");
// "ffv is nil" is not printed
That's correct, releasing an object does not set the pointer to nil even if it is dealloced at that time. Good practice is to always set your pointer to nil after you release it.
You are correct to say that after you released it, it wasn't dealloced because the view still had a retain on it. But that's not how you should be thinking about it. If you want to use that object and you want it to be alive, retain it. Doesn't matter who else is retaining it. Your object has nothing to do with those other objects. You want it, retain it. You're done with it, release it and set your pointers to nil. If you don't set it to nil and everyone else also released it, you will have a dangling pointer to an object that was dealloced, and that will cause you a crash and much grievance.
so this:
[ffv release]; // you can release it now since the view has ownership of ffv
ffv = nil; // you released it, so that means you don't want it anymore, so set the pointer to nil
if you still want to use it, don't release it until you're done with it.
Well, I'm not sure 'losing' ownership is the right term. In Objective-C you have to carefully marshal your ownership of the object. If you create or retain an object, you are responsible for releasing it (either directly or via an autorelease pool). When you call release however, you don't lose a reference to the object, if something else has retained it, it will still be in memory, and your pointer will still potentially point to it.
You have a pointer ffv which is just a pointer to some memory, and you have the object which is created in the first line that ffv points to.
By calling release, you are stating that you no longer require the ponter ffv to point to a valid object, that in this context you would be happy for the object to be deallocated. The pointer still points to that bit of memory, and it is still there because its retain count was increased by assigning it to the view.
The line [ffv testMethod] is in danger of not working, as it follows the release and may not point to a valid object. It only works because something else is keeping it alive. ffv still has the same address value that it had when it was first assigned.
So in order:
Your deduction is correct.
Not really.
You shouldn't use ffv after the release call. You have no guarantee that the object is going to be there for you.
These are pointers we are using here, not references like you find in Java or C#. You have to marshal your ownership of the object, you create it, have some pointers to it and by careful management of retain and release calls you keep it in memory for as long as you need it.
I have a UIView which retain count is three, and I just wanted to know how I can release it so that the retain count is 0?
Thanks in advance.
Did you create it with +alloc or +new, or with a method that has the word "copy" in its name? Did you send it a -retain message? If not, you don't own it and must not release it. And stop looking at the retain count; doing so only serves to over-complicate what is actually a very simple set of rules for memory management.
Never use retain counts to debug. The frameworks do some crazy stuff behind the scenes and can frequently cause your retain counts to be very different from what you think they should be. Release whenever you should and don't worry with it beyond that.
As stated in the official documentation for -retainCount,
Important: This method is typically of no value in debugging memory management issues. Because any number of framework objects may have retained an object in order to hold references to it, while at the same time autorelease pools may be holding any number of deferred releases on an object, it is very unlikely that you can get useful information from this method.
Don’t rely on -retainCount. Other objects may be retaining your object without you knowing it, and autoreleased objects might give you a wrong impression of the actual retain count.
[object release];
but the retain count is probably 3 because the view is currently in use. You only need to release it once. Whatever else is using it (like the parent view or an array) will release it when it is no longer needed. Do you perhaps need to remove it from the parent view?
Sometimes I see developers use:
ClassA *obj = [[ClassA alloc]...]autorelease];
Why does this sometimes autorelease the objects and sometimes not?
Or is this an error?
Autorelease just means "this will be released at a later date". If you autorelease something three times, it will be released three times, later.
Instead of guessing, read through and understand the Memory Management Programming Guide before you do anything else. It will save you a ton of time and frustration.
You also asked, in the comments to another answer, why people use autorelease to begin with. Autorelease is necessary for a common case where you have to create a new object and return it. By the other rules of memory management, you would need to release the object at some point or it would be leaked, but if you release it before you return it, it will go away immediately.
There are two ways that you could deal with this: a) to have these methods return a new object which the caller is expected to release when it is done with the object, and b) to make sure that the object is released as soon as the caller is done with it.
The way Cocoa and Cocoa Touch handle this situation by convention is option b): to use autorelease pools, because you won't have to keep track of ownership except for the objects you explicitly create. Some people use autorelease for nearly everything instead of release, and this is a bit more inefficient than release, but more importantly it hides any useful distinctions you might make between release and autorelease when you're trying to read, navigate and understand the code.
As I said earlier, please read the Memory Management Programming Guide and all these questions will be answered. Better yet, if you just follow the simple rules listed in there, you won't have to think deep thoughts about every combination; there'll be a general rule to follow for your situation and the rule will work.
There's no error with autorelease - it just sets the object to be automatically released at a later time, which you don't necessarily have control over.
I try to always explicitly release my objects, unless I don't have a choice such as when returning a newly alloced/init-ed object from a method.
Actually when you create an object by using the following statement,
ClassA *obj = [[ClassA alloc]init ...];
Then you need to release that object, other wise it will be remain in that app and not useful by any other objects. Then the memory will be wasted.
So we have to release the object by [obj release];
in case some time we can't release at specific time. So we will put autorelease. If we do like that then the NSAutoReleasePool handle the release operation.
We can use any of the above.
Reagrds,
Satya.
No, it's not an error at all. Autorelease means that the object will be released at the end of the current Autorelease pool.
Look at your main.m class file (Every Cocoa/Cocoa Touch project gets it). You will see it has a method that looks a bit like this:
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc]init];
UIApplicationMain(.....);
[pool release];
UIApplicationMain starts your app's run loop. When you reach the end of it, (The application is closed) anything in the autorelease pool is dumped.
Now, if you impliement multithreading and you create your own Autorelease pools within the pool, you'll release the object when that pool is released.
The documentation on the method is here: http://developer.apple.com/library/mac/documentation/Cocoa/Reference/Foundation/Protocols/NSObject_Protocol/Reference/NSObject.html#//apple_ref/occ/intfm/NSObject/autorelease
I'm just beginning to have a look at Objective-C and Cocoa with a view to playing with the iPhone SDK. I'm reasonably comfortable with C's malloc and free concept, but Cocoa's references counting scheme has me rather confused. I'm told it's very elegant once you understand it, but I'm just not over the hump yet.
How do release, retain and autorelease work and what are the conventions about their use?
(Or failing that, what did you read which helped you get it?)
Let's start with retain and release; autorelease is really just a special case once you understand the basic concepts.
In Cocoa, each object keeps track of how many times it is being referenced (specifically, the NSObject base class implements this). By calling retain on an object, you are telling it that you want to up its reference count by one. By calling release, you tell the object you are letting go of it, and its reference count is decremented. If, after calling release, the reference count is now zero, then that object's memory is freed by the system.
The basic way this differs from malloc and free is that any given object doesn't need to worry about other parts of the system crashing because you've freed memory they were using. Assuming everyone is playing along and retaining/releasing according to the rules, when one piece of code retains and then releases the object, any other piece of code also referencing the object will be unaffected.
What can sometimes be confusing is knowing the circumstances under which you should call retain and release. My general rule of thumb is that if I want to hang on to an object for some length of time (if it's a member variable in a class, for instance), then I need to make sure the object's reference count knows about me. As described above, an object's reference count is incremented by calling retain. By convention, it is also incremented (set to 1, really) when the object is created with an "init" method. In either of these cases, it is my responsibility to call release on the object when I'm done with it. If I don't, there will be a memory leak.
Example of object creation:
NSString* s = [[NSString alloc] init]; // Ref count is 1
[s retain]; // Ref count is 2 - silly
// to do this after init
[s release]; // Ref count is back to 1
[s release]; // Ref count is 0, object is freed
Now for autorelease. Autorelease is used as a convenient (and sometimes necessary) way to tell the system to free this object up after a little while. From a plumbing perspective, when autorelease is called, the current thread's NSAutoreleasePool is alerted of the call. The NSAutoreleasePool now knows that once it gets an opportunity (after the current iteration of the event loop), it can call release on the object. From our perspective as programmers, it takes care of calling release for us, so we don't have to (and in fact, we shouldn't).
What's important to note is that (again, by convention) all object creation class methods return an autoreleased object. For example, in the following example, the variable "s" has a reference count of 1, but after the event loop completes, it will be destroyed.
NSString* s = [NSString stringWithString:#"Hello World"];
If you want to hang onto that string, you'd need to call retain explicitly, and then explicitly release it when you're done.
Consider the following (very contrived) bit of code, and you'll see a situation where autorelease is required:
- (NSString*)createHelloWorldString
{
NSString* s = [[NSString alloc] initWithString:#"Hello World"];
// Now what? We want to return s, but we've upped its reference count.
// The caller shouldn't be responsible for releasing it, since we're the
// ones that created it. If we call release, however, the reference
// count will hit zero and bad memory will be returned to the caller.
// The answer is to call autorelease before returning the string. By
// explicitly calling autorelease, we pass the responsibility for
// releasing the string on to the thread's NSAutoreleasePool, which will
// happen at some later time. The consequence is that the returned string
// will still be valid for the caller of this function.
return [s autorelease];
}
I realize all of this is a bit confusing - at some point, though, it will click. Here are a few references to get you going:
Apple's introduction to memory management.
Cocoa Programming for Mac OS X (4th Edition), by Aaron Hillegas - a very well written book with lots of great examples. It reads like a tutorial.
If you're truly diving in, you could head to Big Nerd Ranch. This is a training facility run by Aaron Hillegas - the author of the book mentioned above. I attended the Intro to Cocoa course there several years ago, and it was a great way to learn.
If you understand the process of retain/release then there are two golden rules that are "duh" obvious to established Cocoa programmers, but unfortunately are rarely spelled out this clearly for newcomers.
If a function which returns an object has alloc, create or copy in its name then the object is yours. You must call [object release] when you are finished with it. Or CFRelease(object), if it's a Core-Foundation object.
If it does NOT have one of these words in its name then the object belongs to someone else. You must call [object retain] if you wish to keep the object after the end of your function.
You would be well served to also follow this convention in functions you create yourself.
(Nitpickers: Yes, there are unfortunately a few API calls that are exceptions to these rules but they are rare).
If you're writing code for the desktop and you can target Mac OS X 10.5, you should at least look into using Objective-C garbage collection. It really will simplify most of your development — that's why Apple put all the effort into creating it in the first place, and making it perform well.
As for the memory management rules when not using GC:
If you create a new object using +alloc/+allocWithZone:, +new, -copy or -mutableCopy or if you -retain an object, you are taking ownership of it and must ensure it is sent -release.
If you receive an object in any other way, you are not the owner of it and should not ensure it is sent -release.
If you want to make sure an object is sent -release you can either send that yourself, or you can send the object -autorelease and the current autorelease pool will send it -release (once per received -autorelease) when the pool is drained.
Typically -autorelease is used as a way of ensuring that objects live for the length of the current event, but are cleaned up afterwards, as there is an autorelease pool that surrounds Cocoa's event processing. In Cocoa, it is far more common to return objects to a caller that are autoreleased than it is to return objets that the caller itself needs to release.
Objective-C uses Reference Counting, which means each Object has a reference count. When an object is created, it has a reference count of "1". Simply speaking, when an object is referred to (ie, stored somewhere), it gets "retained" which means its reference count is increased by one. When an object is no longer needed, it is "released" which means its reference count is decreased by one.
When an object's reference count is 0, the object is freed. This is basic reference counting.
For some languages, references are automatically increased and decreased, but objective-c is not one of those languages. Thus the programmer is responsible for retaining and releasing.
A typical way to write a method is:
id myVar = [someObject someMessage];
.... do something ....;
[myVar release];
return someValue;
The problem of needing to remember to release any acquired resources inside of code is both tedious and error-prone. Objective-C introduces another concept aimed at making this much easier: Autorelease Pools. Autorelease pools are special objects that are installed on each thread. They are a fairly simple class, if you look up NSAutoreleasePool.
When an object gets an "autorelease" message sent to it, the object will look for any autorelease pools sitting on the stack for this current thread. It will add the object to the list as an object to send a "release" message to at some point in the future, which is generally when the pool itself is released.
Taking the code above, you can rewrite it to be shorter and easier to read by saying:
id myVar = [[someObject someMessage] autorelease];
... do something ...;
return someValue;
Because the object is autoreleased, we no longer need to explicitly call "release" on it. This is because we know some autorelease pool will do it for us later.
Hopefully this helps. The Wikipedia article is pretty good about reference counting. More information about autorelease pools can be found here. Also note that if you are building for Mac OS X 10.5 and later, you can tell Xcode to build with garbage collection enabled, allowing you to completely ignore retain/release/autorelease.
Joshua (#6591) - The Garbage collection stuff in Mac OS X 10.5 seems pretty cool, but isn't available for the iPhone (or if you want your app to run on pre-10.5 versions of Mac OS X).
Also, if you're writing a library or something that might be reused, using the GC mode locks anyone using the code into also using the GC mode, so as I understand it, anyone trying to write widely reusable code tends to go for managing memory manually.
As ever, when people start trying to re-word the reference material they almost invariably get something wrong or provide an incomplete description.
Apple provides a complete description of Cocoa's memory management system in Memory Management Programming Guide for Cocoa, at the end of which there is a brief but accurate summary of the Memory Management Rules.
I'll not add to the specific of retain/release other than you might want to think about dropping $50 and getting the Hillegass book, but I would strongly suggest getting into using the Instruments tools very early in the development of your application (even your first one!). To do so, Run->Start with performance tools. I'd start with Leaks which is just one of many of the instruments available but will help to show you when you've forgot to release. It's quit daunting how much information you'll be presented with. But check out this tutorial to get up and going fast:
COCOA TUTORIAL: FIXING MEMORY LEAKS WITH INSTRUMENTS
Actually trying to force leaks might be a better way of, in turn, learning how to prevent them! Good luck ;)
Matt Dillard wrote:
return [[s autorelease] release];
Autorelease does not retain the object. Autorelease simply puts it in queue to be released later. You do not want to have a release statement there.
My usual collection of Cocoa memory management articles:
cocoa memory management
There's a free screencast available from the iDeveloperTV Network
Memory Management in Objective-C
NilObject's answer is a good start. Here's some supplemental info pertaining to manual memory management (required on the iPhone).
If you personally alloc/init an object, it comes with a reference count of 1. You are responsible for cleaning up after it when it's no longer needed, either by calling [foo release] or [foo autorelease]. release cleans it up right away, whereas autorelease adds the object to the autorelease pool, which will automatically release it at a later time.
autorelease is primarily for when you have a method that needs to return the object in question (so you can't manually release it, else you'll be returning a nil object) but you don't want to hold on to it, either.
If you acquire an object where you did not call alloc/init to get it -- for example:
foo = [NSString stringWithString:#"hello"];
but you want to hang on to this object, you need to call [foo retain]. Otherwise, it's possible it will get autoreleased and you'll be holding on to a nil reference (as it would in the above stringWithString example). When you no longer need it, call [foo release].
The answers above give clear restatements of what the documentation says; the problem most new people run into is the undocumented cases. For example:
Autorelease: docs say it will trigger a release "at some point in the future." WHEN?! Basically, you can count on the object being around until you exit your code back into the system event loop. The system MAY release the object any time after the current event cycle. (I think Matt said that, earlier.)
Static strings: NSString *foo = #"bar"; -- do you have to retain or release that? No. How about
-(void)getBar {
return #"bar";
}
...
NSString *foo = [self getBar]; // still no need to retain or release
The Creation Rule: If you created it, you own it, and are expected to release it.
In general, the way new Cocoa programmers get messed up is by not understanding which routines return an object with a retainCount > 0.
Here is a snippet from Very Simple Rules For Memory Management In Cocoa:
Retention Count rules
Within a given block, the use of -copy, -alloc and -retain should equal the use of -release and -autorelease.
Objects created using convenience constructors (e.g. NSString's stringWithString) are considered autoreleased.
Implement a -dealloc method to release the instancevariables you own
The 1st bullet says: if you called alloc (or new fooCopy), you need to call release on that object.
The 2nd bullet says: if you use a convenience constructor and you need the object to hang around (as with an image to be drawn later), you need to retain (and then later release) it.
The 3rd should be self-explanatory.
Lots of good information on cocoadev too:
MemoryManagement
RulesOfThumb
As several people mentioned already, Apple's Intro to Memory Management is by far the best place to start.
One useful link I haven't seen mentioned yet is Practical Memory Management. You'll find it in the middle of Apple's docs if you read through them, but it's worth direct linking. It's a brilliant executive summary of the memory management rules with examples and common mistakes (basically what other answers here are trying to explain, but not as well).